1. Technical Field
The present disclosure relates to a calibration device and a calibration method for correcting carrier leakage that occurs in a modulated transmission wave in a quadrature modulator.
2. Description of the Related Art
Recently, for wireless local area networks (LANs), the standardization of the Institute of Electrical and Electronics Engineering, Inc. (IEEE) 802.11ad communication specifications has been promoted. In IEEE 802.11ad, for example, phase modulation, quadrature amplitude modulation, or orthogonal frequency division multiplexing (OFDM) has been adopted as a modulation method for data transmission. When such a modulation method is used, to convert a baseband signal to a microwave or millimeter-wave modulated signal, a quadrature modulator is used.
To suppress an increase in circuit size, for example, it is desirable to perform quadrature modulation in a millimeter-wave frequency band. However, because a millimeter-wave is a radio frequency signal, crosstalk tends to occur in the inside of a circuit of a transmitting device (for example, a quadrature modulator), and carrier leakage tends to occur in the radio frequency signal after the quadrature modulation. Because carrier leakage is an unwanted component in reception by a receiving device, the detection accuracy for a received signal is degraded. Therefore, when the transmitting device generates a radio frequency signal, it is necessary to suppress (correct) carrier leakage that occurs in the radio frequency signal in the quadrature modulator.
Known techniques in the related art for correcting carrier leakage that occurs in a radio frequency signal in a quadrature modulator include, for example, techniques in Japanese Unexamined Patent Application Publication No. 8-213846 and Japanese Unexamined Patent Application Publication No. 2012-85100.
A transmitting device described in Japanese Unexamined Patent Application Publication No. 8-213846 supplies a signal to a quadrature modulator, and performs envelope detection of an output from the quadrature modulator, thereby obtaining an envelope amplitude. Then, the transmitting device inputs, to the quadrature modulator, a distortion correction signal by which a fluctuation amount of the envelope amplitude is made smaller, and corrects carrier leakage.
A wireless communication device described in Japanese Unexamined Patent Application Publication No. 2012-85100 has a setting table in which, for each value of transmission power of a modulated signal after quadrature modulation is performed, the transmission power value is associated with change amounts of direct current components of I and Q signals; for the change amounts, a carrier leakage amount included in the modulated signal having the transmission power becomes smaller than or equal to a predetermined value. By using the setting table, the wireless communication device adjusts, by change amounts corresponding to transmission power indicated by a power signal, direct current components of I and Q signals before quadrature modulation is performed.